This work centers on the high molecular weight, high mobility group (HMG), nonhistone chromatin proteins. We have previously shown that HMG-1 and HMG-2 occur in cultured hepatoma cells in much higher amounts than in adult rat liver and that both proteins possess preferential affinity for single-stranded DNA. We discovered a three-domain organization of HMG-1 and HMG-2. Two of the domains are sequence homologous, globular, DNA-binding domains. The third domain is probably a random coil that binds to histones. The proteins are thus peculiarly adapted to interact through separate domains with both DNA and histones. We recently searched for nonhistone chromosomal proteins that bind to single-stranded DNA in the presence of dextran sulfate. This polyanionic reagent has been suggested by other workers to dissociate proteins that bind nonspecifically to single-stranded DNA. Dextran sulfate-resistant proteins, several of which we have now identified in cultured L cells, are presumed to have a more specific interaction with single-stranded DNA and are more likely to interact with single-stranded DNA in vivo. Efforts are under way to purify individual dextran sulfate-resistant single-stranded DNA-binding proteins. We also made major progress in two-dimensional gel electrophoresis of HMG proteins. In addition, we obtained improved resolution of multiple forms and demonstrated a requirement for removing histones from the HMG proteins before two-dimensional electrophoretic analysis. Using our two-dimensional gel methodologies, we have examined HMG proteins from a variety of species and organs. The conclusion from these studies is that variation in multiple forms is largely a species-specific rather than a tissue-specific phenomenon. (K)